This invention relates to a device for the electrical connection of a filament lamp of a given nominal voltage, in a motor vehicle having an electrical power supply network which is under a network voltage higher than the nominal lamp voltage.
As the number of items of electrical equipment in a motor vehicle becomes higher and higher, an increase in the available electrical power is becoming more and more necessary. This leads to an increase in the network voltage of the vehicle electrical system, in order to avoid the occurrence of excessively high currents in the system. The unidirectional network voltage is usually 12 volts in current automobiles such as family cars or 24 volts in heavy goods vehicles (trucks). Sometimes, this voltage must increase substantially, in particular to reach as high as 42 volts. Technical considerations appropriate to the automotive field, linked to the size of the tungsten filament of the lamp, make it improbable that filament lamps directly supplied at 42 volts can or will be made. Filament lamps arranged to function at a voltage of 12 volts will still be in service even where the network voltage of the vehicle is higher than the nominal lamp voltages. Various solutions have already been proposed to overcome this problem.
In this connection, systems are known with a centralized architecture which comprises a DC/DC converter which supplies a reduced unidirectional voltage, for example at 12 volts, from a higher input voltage which may for example be 42 volts. This solution makes it necessary to provide a 12 volt battery as well as the converter. This arrangement results in relatively high cost and a significant space requirement.
It is also known to provide apparatus with an independent electrical control unit which enables the unidirectional voltage applied to the lamps to be regulated by a pulse width modulated (PWM) control signal. Such a control unit is remote from the various lamps which are to be supplied, and must be connected to the fittings that carry those lamps, the connection being made by electrical wires. This arrangement has various drawbacks, in particular the size of the control unit to be provided, the power of which must be high enough to supply all of the appropriate lamps in the vehicle. Cabling must be provided between tho control unit and the lamps. In addition, it is difficult with this arrangement to satisfy the requirements for electromagnetic compatibility, due to the emission of parasitic radiation set up by the pulse width modulated control signal.
A primary object of the invention is to provide a device for the connection of a filament lamp, having a nominal voltage lower than that of the power supply network, but which responds better than at present to the various requirements found in practice. In particular, it is desirable that the connecting device enables a double network to be avoided in the cable bundle. It is also desirable that it shall facilitate adaptation to a higher voltage and provide proper screening against electromagnetic radiation.
According to the invention, a device for connecting a filament lamp, having a given nominal voltage, in a motor vehicle equipped with a network for power supply at a voltage greater than the nominal voltage of the lamp, is characterised in that the device incorporates an electronic circuit including components for reducing the voltage from the network voltage to a voltage close or equal to the nominal voltage of the lamp, the device also incorporating means for making the electrical connection of the lamp.
Preferably, the device further provides mechanical fastening for the lamp.
The device may constitute a connector for the lamp, for plug-in connection of terminal tags of the lamp. Alternatively, it may constitute a lamp holder socket.
The device preferably includes a casing of plastics material in which the electronic circuit for reducing the voltage is placed, the casing being provided with electrical connecting contacts or sockets for connection with the filament lamp.
According to a preferred feature of the invention, the casing incorporates a metallic cooling element for the electronic components of the circuit, the said metallic element also providing screening for purposes of electromagnetic compatibility.
Where the device is a connector comprising a casing of plastics material containing the electronic voltage reduction circuit and arranged to receive and provide electrical connection to plug-in tags of the lamp, then, according to a preferred feature of the invention, the casing includes two projecting elements which are open in a face remote from the casing, each said projecting element having an internal metallic tongue, the said tongues being connected to the output terminal of the voltage reduction circuit, the tongues being arranged to receive and mechanically hold the tags of the lamp, besides making the electrical connection thereof. Preferably, the projecting elements completely surround the tongues, whereby to insulate the said tongues electrically.
Where the device comprises a lamp holder socket in the form of a sleeve, for example for bayonet or screw-type fastening of the lamp, and has a casing of plastics material containing the electronic voltage reduction circuit, the sleeve is joined to one face of the casing which includes, on another face, a projecting element open at the end thereof remote from the circuit, with two electrical contacts being mounted within the said projecting element.
Preferably, the said circuit is arranged to establish pulse width modulation and to provide an output voltage close or equal to the nominal voltage of the lamp. The said circuit preferably then comprises an integrated circuit which is arranged to produce a pulse width modulated signal on its output, together with components for creating a regulated voltage suitable for proper operation of the integrated circuit.
Preferably, the frequency of the pulse width modulated signals is chosen to be large enough to prevent any perception by the human eye of any variation in light intensity from the lamp, having regard to retinal persistence. In particular, the frequency is in the range between 60 Hz and 500 Hz.
The connecting device according to the invention enables additional connections to be avoided with respect to a current 12 volt or 24 volt network or a future 42 volt network. In this connection, the network voltage is 42 volts, while the nominal voltage of the lamp is 12 or 24 volts.
Preferably, the said components of the circuit are soldered on a printed circuit, as are the output terminals and the input wires.
Electromagnetic compatibility is facilitated by the fact that the lamp filament is very close to the control means which reduce the voltage to the lamp input value.